DESCRIPTION (Applicant's Description): The objective of this project is to define the role of adaptor proteins in malignant transformation. Adaptor proteins are unique signaling molecules which, although devoid of any intrinsic enzymatic activity, function mainly by allowing the interconnection of other molecules in various signaling pathways through protein- protein interactions. While mutation of adaptor proteins in the laboratory can cause cells to develop malignant characteristics, a similar process appears to occur in vivo through adaptor protein interactions with various oncogenic proteins. The specific hypothesis of this project is that alteration of adaptor proteins by oncogenic tyrosine kinases is an important mechanism by which oncogenes induce malignant transformation. This project will focus on the adaptor protein CRKL which is constitutively expressed in a number of hematopoietic and non-hematopoietic cells in an unphosphorylated state. Cells which contain the BCR-ABL oncogenic tyrosine kinase, seen in all leukemic cells containing the Philadelphia-1 chromosome abnormality, express high levels of phosphorylated CRKL. The facts that CRKL binds directly to BCR-ABL, that CRKL is the most prominent tyrosine phosphorylated protein in BCR-ABL positive cells, and that significant CRKL phosphorylation occurs only in cells which express BCR-ABL are strongly suggestive that alteration of this adaptor protein through phosphorylation plays a unique role in the malignant transformation of BCR-ABL positive cells. Using several mutated forms of the CRKL protein, the specific alms of this study will be: 1) To define the role of CRKL in malignant transformation induced by BCR-ABL by determining the role of CRKL tyrosine phosphorylation in the oncogenic process and whether downstream effectors and signal transduction pathways utilized by CRKL are affected by CRKL phosphorylation; 2) To determine if CRKL in any form is transforming in the absence of BCR-ABL and develop a model of how CRKL functions with regard to its own phosphorylation; 3) To explore the role of CRKL in other solid tumor systems like lung cancer, breast cancer, and colorectal cancer which have been associated with alterations in tyrosine kinase signaling pathways.